The invention relates to an oscillator circuit comprising a feedback amplifier circuit having a power supply terminal, a resonant circuit comprising a crystal and a differential amplifier which is coupled to a bias current source. A crystal oscillator circuitt of this type is known, inter alia, from U.S. Pat. No. 4,574,257.
The starting behavior of crystal oscillators upon switching on the power supply voltage is greatly dependent on thew ay in which a sufficient quantity of energy is supplied to the crystal to achieve resonance. The quantity of supplied energy is mainly determined by thew ay in which the power upply voltage is built up, which depnds, inter alia, on the decoupling capacitors for the power supply voltage used inthe circuit, on the implementation of D.C. bias circuits in the circuit and on the noise applied to the crystal. Consequently, crystal oscillators have an undenfined, non-reproducible starting behaviour in practice.
However, such an undenfined, non-reproducible starting behaviour causes difficulties when testing crystal oscillators. The total test period of, for example, integrated circuits is approximately 4 to 5 seconds depend on their volume. However, if these integrated circuits comprise a crystal oscillator, the total test period is extended by the period which is required to make the oscillator resonate. Sine this period, as stated above, cannot be laid down or is not known in advance, a waiting time with a safety margin of the order of approximately 0.5 sec. is generally used in practice. This means that the total test period of integrated circuits comprising a crystal oscillator is approximately 10% longer than that of corresponding circuits without crystal oscillators. It will therefore be evident that the total time which is required to manufacture integrated circuits comprising a crystal oscillator is also longer. A reduction of the total manufacturing period of such integrated circuits may not only have a cost saving effect in practice, but it also provides the possibility of increasing the total production.
It is known from IBM Technical Disclosure Bulletin, Vol. 21, No 2, July 1978, pp. 492-493 that the starting behaviour of a crystal oscillator circuit can be improved by stepped excitation of the resonant circuit. An oscillator is shown in which the excitation is realised by stepped interruption of a current through a coil which forms part of the resonant circuit. The sudden interruption of the current causes a change of the voltage across the coil so that the oscillator circuit starts. However, this known method of excitation is only effective in crystal oscillator circuits in which the resonant circuit incorporates a coil and it is not suitable for an oscillator circuit of the type described in the opening paragraph.